502 when running integration tests

When running a test application factory generated client, i receive 502 bad gateway, here Startup.cs

public void ConfigureServices(IServiceCollection services)
        {
            services.AddRouting();

            // Add the reverse proxy to capability to the server
            var proxyBuilder = services.AddReverseProxy();
            // Initialize the reverse proxy from the "ReverseProxy" section of configuration
            proxyBuilder.LoadFromConfig(configuration.GetSection("ReverseProxy"));
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapReverseProxy();
            });
        }

and appsettings.json ReverseProxy settings

"Routes": {
      "TestMe": {
        "ClusterId": "all",
        "Match": {
          "Path": "test-proxy"
        }
      }
    },
    "Clusters": {
      "all": {
        "Destinations": {
          "all/destination1": {
            "Address": "https://127.0.0.1/hello"
          }
        }
      }
    }

with this app factory

 public class TestAppFactory : WebApplicationFactory<Startup>
    {
        protected override void ConfigureWebHost(IWebHostBuilder builder)
        {
            builder.UseContentRoot(System.IO.Directory.GetCurrentDirectory())
                .UseUrls("http://*:5000/")
                .ConfigureAppConfiguration(cfg =>
                    cfg.AddJsonFile("appsettings.json").AddEnvironmentVariables()
                );
        } 
    }

using Stubbery;

public class ExampleTest : IClassFixture<TestAppFactory>
    {
        private readonly TestAppFactory testAppFactory;

        public ExampleTest(TestAppFactory testAppFactory)
        {
            this.testAppFactory = testAppFactory;
        }

        [Fact]
        public async Task ExampleGet_Called_HelloReturned()
        {
            using var stub = new ApiStub();

            stub.Get("/hello/test-proxy", (cxt, args) => "Hello Proxy!");

            stub.Start();

            var client = testAppFactory.CreateClient();
            
            var response = await client.GetAsync("/test-proxy");

            response.EnsureSuccessStatusCode();

            var content = await response.Content.ReadAsStringAsync();

            Assert.Equal("Hello Proxy!", content);
        }
    }

[donaldgray]

I realise this is closed but if any one comes across this: IProxyHttpClientFactory has been renamed to IForwarderHttpClientFactory.

[ghost]

While this issue is closed, The library has been updated since and it took me a while to figure out a solution, therfor, ill share my own solution:

public class TestHostFixture
    {
        private static IWebHost _host;
        public TestHostFixture() => _host = Api.Program.CreateWebHostBuilder(new string[] { })
            .ConfigureAppConfiguration((context, configurationBuilder) => configurationBuilder
                .AddJsonFile("appsettings.tests.json"))
            .ConfigureTestServices(serviceCollection =>
                serviceCollection.AddSingleton<IForwarderHttpClientFactory, CustomForwarderHttpClientFactory>())
            .UseTestServer()
            .Start();

        public HttpClient GetTestClient() => _host.GetTestClient();

        public class CustomForwarderHttpClientFactory : IForwarderHttpClientFactory
        {
            public HttpMessageInvoker CreateClient(ForwarderHttpClientContext context) =>
                new HttpMessageInvoker(new TestProxyHandler(new HttpClientHandler()), false);

            public class TestProxyHandler : DelegatingHandler
            {
                public TestProxyHandler(HttpMessageHandler handler) : base(handler) { }
                protected override async Task<HttpResponseMessage> SendAsync(
                    HttpRequestMessage request, CancellationToken cancellationToken) =>
                    await _host
                        .GetTestClient()
                        .SendAsync(request, cancellationToken);
            }
        }
    }

P.S I still believe this a limitation of the library and should be resolved one way or another. The hack above, while it works, should not be required, since (at least in the case above), we are running a duplicate of the Server.